Adapt acceleration/deceleration depending on current speed

[quajak]

getAdaptedThrottle currently works both when accelerating and decelerating.

[jschirrmacher]

If we would do so, the car could easily overturn. In fact, it already does so, if we break from 100% throttle or even if we use a negative acceleration from a high positive speed. So, it should be more desirable to actually decrease the deceleration speed.

Maybe we should make it depend on the actual speed of the car.